IPv6: enable IPv6 link local routing for local network

IPv6 link local is an automatically configured address. The routing should be
activated regardless of the DHCP service.

Bug: 126063997
Test: Unit tests - atest FrameworksNetTests
Test: Unit tests - atest FrameworksWifiTests
Test: * DUT1 enables hotspot
      * DUT2 associates to DUT1
      * use ping6 to verify the local link on both side
Test: * DUT1 creates a P2P group
      * DUT2 joins this group as a legacy client
      * use ping6 to verify the local link on both side

Change-Id: I102bf29e8da14808097e43c840d74d41b6b3758f